The present invention relates to spot welding machines, and in particular to such a machine capable of heavy-duty spot welding operations on surface contoured workpieces in response to a set of instruction data.
In automobile manufacturing plants, it is customary to produce a single model of cars on a single production line since there do not exist multi-purpose machine tools which make it possible to produce different models on the same production line. It is desirable however to mass-produce a variety of models of cars on a single production line in terms of efficiency, space savings and equipment cost.
Attempts have hitherto been made to employ industrial robot systems because of their capability to repeatedly position an object according to a prescribed pattern under the direction of instruction data including tool position data and operating speed.
Conventional robot systems comprise a main body installed on the ground and an arm pivoted at one end to the main body for holding a tool at the other end. In some machining operations such as spot welding, a substantial amount of force is applied to a workpiece and the resulting reaction force must be borne by the entire structure. However, the conventional robot system is not capable of bearing such reaction force and deformation tends to occur.
To avoid such deformation conventional automatic spot welding machines are provided with upper and lower arms in an X-shaped or C-shaped configuration to distribute the reaction force components between the upper and lower arms. However, large sized workpieces would require the machine to extend its arm a substantial distance from its standing position. Furthermore, the conventional machine employs upper and lower electrode tips between which the workpiece is pressure-contacted and current is passed between the electrode tips. Therefore, when the machine arm extends a substantial distance from its standing position, the secondary impedance of the power transformer would increase requiring additional electrical power and resulting in non-uniform spot welds.